I don't know if you knew but there is a Hobie store on R58 in Ginowan. I haven't been in but I'm sure they can order parts if you are stuck though to be honest I heard they weren't so good price or service wise.
I also came across an Okinawan Hobie group though they don't seem to be active - I tried contacting them a while ago but got no reply.http://hobiecat.ti-da.net/
I keep my Hobie up at a place on the Motobu/Nakijin peninsula. I know the manager of a pension/holiday let place there that has direct beach access. I can leave it there and get in and out of the water in minutes which is perfect for those times when you can skip out of work a couple of hours early when the conditions are just right and go for a quick sail. One of the cottages there is owned by an English guy who is based in Tokyo but spends the holidays there. He has a couple of dinghys and windsurfers so when he's here we can go out together but other than that there aren't any others out there. I've yet to have any problems but it's definitely wise to pick the days carefully. My Hobie is at best guess 30 yrs old and although I recently replaced all the standing rigging you never know when something might just give up and I could be demasted and floating out on an offshore to who knows where! I keep my cell with me at all times and always let the manager of the place know I'm going out and how long I'll be. You'll be in a better fettle having 3 modes of propulsion should something go wrong but it's a pain to paddle a cat!
I've seen the TI at Kadena too but as for other Hobie sightings here they are few and far between. I've seen some parked up on the beach at Yaka/Igei on the east coast (I think they must belong to the Hobie group I mentioned earlier looking at some of the photos on their blog). A small Cafe down nr Kin Power station has 3 or 4 but they never seem to be used. Some of the resort hotels have rentals (Sun Marina, Rennaisance, Busens, Resonex in Nago). It seems locals just don't get sailing preferring put put fishing boats or jet skis. Lots of kayaks around of course.
That's a great spot you are in - know it well - perfect for the TI. There are some great spots all along from Zampa to Maeda and then on towards Coconut Moon for jumping out and snorkeling.